1. Field of the Invention
This invention relates to photoselective metallization of non-conductive plastic base materials. More specifically, it relates to the manufacture of additive printed circuits by electrolessly plating catalytic images on adhesion promoted surfaces of insulating base materials.
2. Description of the Prior Art
In "resist imaging" fully additive processes which are commonly employed today for producing printed circuits on resinous insulating base materials, a conductor pattern is formed by electrolessly plating metal on an adhesion promoted surface of the resinous insulating base material. A plating resist is applied before or after adhesion promotion, onto the background, which is the area free of electrolessly deposited conductors on the printed circuit board. In order to obtain an adhesion promoted surface, the surface of the insulating base material is rendered microporous and hydrophilic. This may be accomplished by plasma or by mechanical means, but is usually achieved by a strong acid or oxidizing solution, or by swelling the surface with a solvent, and then subjecting the pretreated surface to a strong oxidizing agent such as chromic acid. The background is hydrophobic during electroless plating because the surface of the plating resist is hydrophobic.
In "resistless imaging" additive processes, a conductive pattern image, catalytic for electroless deposition, is formed on the adhesion promoted and thus hydrophilic, surface of an insulating base material. Fully additive resistless imaging processes have a hydrophilic background during electroless plating. The presence of a hydrophilic background surface during metal plating renders such background susceptible to extraneous metal plating, which is undesirable. Unlike resist imaging, fully additive processes, which rely upon hydrophobic resist masking layers, resistless systems rely instead on heavily stabilized plating baths and/or periodic "quick etching" during the plating cycle to minimize adherence of extraneous metal deposits.
De Angelo et al., in U.S. Pat. No. 3,562,005 describe a resistless imaging process wherein an adhesion promoted surface is treated with stannous chloride. A conductor image is produced by exposing the surface to hard ultra-violet radiation through a photographic positive of the desired circuit pattern. The exposure converts the stannous moiety between the conductors to a stannic species. Subsequently, the surface is treated with palladium chloride and only the stannous moiety remaining in the conductive pattern reacts with the palladium chloride to form an image catalytic for electroless metal deposition. This process, while capable of reproducing conductor images of very fine detail, never was used in production by Western Electric Co., Inc., the assignee of De Angelo et al. because of defects caused by extraneous metal deposition.
Mansveld and Jans, Plating and Surface Finishing, Vol. 66, page 14 (Jan. 1979); Jonker et al., U.S. Pat. No. 3,674,485; Jans, U.S. Pat. No. 4,451,505; and Lippits et al., U.S. Pat. No. 4,085,285 describe production of a resistless image by illuminating a titanium dioxide coated or filled surface through a photographic negative of the desired pattern to impart a surface charge. The surface charge, in a positive pattern, is treated with a palladium containing solution to provide in the charged areas a resistless image catalytic for electroless metal deposition. Despite substantial investment by the owner of this technology, N.V. Philips Gloeilampenfabrieken ("Philips"), including a full scale manufacturing facility, Philips was unable to resolve the problems related to extraneous metal deposition and this process did not find acceptance as a production method.
Polichette et al. in U.S. Pat. Nos. 3,772,078; 3,930,963; 3,959,547; and 3,994,727, teach a resistless imaging process for producing printed circuits wherein a real image is produced on a surface of an insulating base material which has been treated with a solvent such as dimethylformamide and with chromic acid and/or sulfuric acid to adhesion promote the surface. The adhesion promoted surface is treated with a solution containing copper salts, 2,6-anthraquinone disulfonic acid and a polyol. The treated surface is dried and then exposed to printing light through a photographic negative to produce a real image of copper nuclei which are catalytic for electroless copper plating. The anthraquinone disulfonic acid and unexposed copper salts are removed from the surface by washing. In practice, this process also had extraneous copper deposition, both "flare" (a plume shaped deposit of extraneous copper particles growing above a conductor on the insulating surface of the base material placed vertically in an electroless plating solution) as well as general random formation of extraneous metal deposits in areas between the conductive patterns.
Dafter, U.S. Pat. No. 4,084,023; Dinella et al., U.S. Pat. No. 4,098,922; Madsen, U.S. Pat. No. 4,133,908; Beckenbaugh et al., U.S. Pat. No. 4,167,601; and Beckenbaugh et al., U.S. Pat. No. 4,268,536, all assigned to Western Electric, Inc. ("Western Electric"), describe methods of improving the "contrast" and providing "anti-fogging" in the processes of Polichette et al. Poor contrast and fogging are synonymous with the formation of extraneous metal deposits. Among the aforementioned methods are pre-dips in alkaline chelating solutions and organic acid solutions to improve removal of anthraquinone disulfonic acid and unexposed copper salts; and the use of special electroless plating bath stabilizers to reduce extraneous metal deposition by the plating solution. This process necessitated tight control to prevent extraneous metal deposition.
Ehrich et al., in U.S. Pat. No. 4,262,085, describe another improvement to the process of Polichette et al. directed to the elimination of extraneous metal deposition. After forming a conductor image of copper nuclei, on a surface of a base material, Ehrich et al. exchange the copper nuclei with palladium, rinse, deposit a thin coating of electroless nickel and then electrolessly deposit a copper conductive pattern from a solution stabilized with potassium selenocyanate. Although the Ehrich et al. method was reported to have achieved successful results in pilot plant operation, it was found on a production scale to be unreliable with a tendency not to produce a complete image of the conductive pattern and not to produce freedom from extraneous metal deposits.
The need for a reliable resistless imaging process suitable for the manufacture of printed circuit boards has existed for almost two decades. The substantial efforts in the past around the world by major electronic and chemical companies to develop this process have been unsuccessful. "Vapor Polishing" with methylene chloride has been used to remove scratches from thermoplastics such as polycarbonate, leaving a smooth, glossy finish, e.g., See Resin Lexan.RTM..sup.R Technifact Bulletin (11/82) by General Electric Company, One Plastics Avenue, Pittsfield, Mass. 01201. Vapor polishing also has been used to restore semi-additive printed circuits on polysulfone based material to a transparent state.